The Golden Lily (Bloodlines #2) by Richelle Mead

Sydney Sage is an Alchemist, one of a group of humans who dabble in magic and serve to bridge the worlds of humans and vampires. They protect vampire secrets and human lives.

Sydney would love to go to college, but instead, she’s been sent into hiding at a posh boarding school in Palm Springs, California–tasked with protecting Moroi princess Jill Dragomir from assassins who want to throw the Moroi court into civil war. Formerly in disgrace, Sydney is now praised for her loyalty and obedience, and held up as the model of an exemplary Alchemist.

But the closer she grows to Jill, Eddie, and especially Adrian, the more she finds herself questioning her age-old Alchemist beliefs, her idea of family, and the sense of what it means to truly belong. Her world becomes even more complicated when magical experiments show Sydney may hold the key to prevent becoming Strigoi - the fiercest vampires, the ones who don’t die. But it’s her fear of being just that - special, magical, powerful - that scares her more than anything. Equally daunting is her new romance with Brayden, a cute, brainy guy who seems to be her match in every way. Yet, as perfect as he seems, Sydney finds herself being drawn to someone else - someone forbidden to her.

When a shocking secret threatens to tear the vampire world apart, Sydney’s loyalties are suddenly tested more than ever before. She wonders how she's supposed to strike a balance between the principles and dogmas she's been taught, and what her instincts are now telling her.